1. Field of the Invention
The present invention relates to the checking of turbomachine blades.
After a turbomachine blade is manufactured and before the turbomachine blade is mounted on a rotor disk or a casing, a turbomachine blade is checked, that is to say inspected in order to determine whether this blade manufactured in an industrial process corresponds to a reference blade, that is to say to the theoretically desired blade. This essential check is used to verify the main deviations from the definition and to sanction possible discrepancies in performance.
This check proves to be even more important in the case of engines under development, especially for demonstrators or prototypes under development. This is because geometrical knowledge of the parts used makes it possible to overcome possible prejudicial discrepancies in the understanding of the operation of the turbomachine.
2. Description of Related Art
Various techniques for checking blades are known in the prior art. One essential step common to various checking techniques consists, according to the prior art, in making a three-dimensional recording in the Cartesian coordinates of a plurality of points of an inspected blade. The measurement is performed automatically by means of a device, known to those skilled in the art, comprising a support on which a blade to be measured is immobilized and at least one sensor for measuring the geometrical coordinates at various points on the blade. In a first variant, the support is immobile and the sensor can be moved mechanically. Conversely, in a second variant, the support can be moved mechanically and the sensor is immobile. In a third variant, both the support and the sensor can be moved mechanically.
Document U.S. Pat. No. 5,047,966 describes various techniques for the three-dimensional geometric measurement of a blade. Document U.S. Pat. No. 4,653,011 involves a contact technique in which the end of a sensor comes into contact with the object to be measured. Other techniques, which are contactless, make use of X-ray sources (U.S. Pat. No. 6,041,132) or laser sources (U.S. Pat. No. 4,724,525).
A standard technique for measuring the geometry of successive points is also described in U.S. Pat. No. 5,047,966. The cartesian coordinates of points are recorded in parallel sections of the blade. In the example cited, 840 discrete points are recorded in 28 parallel sections. The number of points may vary according to the desired precision. At the present time, 300 points may be required for a single section. These points on the measured blade are then stored in a memory on a computer recording medium.
To determine the conformity of the blade produced in an industrial process to the desired theoretical blade, on the one hand a model of a reference blade and, on the other hand, acceptable tolerances are provided.
This reference model defines an ideal blade by various geometrical points stored on a computer recording medium. Such a model is illustrated in document EP 1 498 577, which describes a table containing the cartesian coordinates of a reference blade. In this example, a tolerance of ±0.150 inches in a direction normal to the surface of any point on the checked blade is set. A checked blade departing from the reference blade can thus be rejected.
The tolerances may also take into account deviations in translation or in angular orientation, as described in document U.S. Pat. No. 6,748,112, without distinction between more relevant points than others. The prior art therefore relies on exclusively geometrical criteria for validating or rejecting a checked blade.
FIG. 1 shows schematically a blade section 10. According to the prior art, a tolerance 4 determined according to the geometrical deviation between the reference blade and the measured blade makes it possible to define the extreme deviations 2 and 3 that this checked blade can take. These deviations 2 and 3 define a space in which the checked blade 1 must lie in order not to be rejected. To carry out the measurement, the blade is preferably immobilized on a support.
The requirements in terms of desired precision at the present time are such that the mass of information, essentially consisting of the cartesian coordinates of all the measured points in a plurality of blade sections becomes considerable and it is difficult to synthesize it. Moreover, the geometrical deviations cannot be directly interpreted from an aerodynamics standpoint.